The long-term objective is to understand how a cell maintains the integrity of its genetic information, more particularly how it protects the DNA from damaging agents. The source of damage that will be primarily studied will be UV irradiation. The prokaryotic E. coli with its bacteriophages and the eukaryotic alga Chlorella with its chlorellophages are the organisms that will be studied because they present great technical advantages. The RecA protein of E. coli, which is both a recombinase and a so-called protease, plays a key multifaceted role in repair of damaged DNA. An understanding of all aspects of this role is one of the main goals of the research program. An activated form of the RecA protein takes part in both repair and mutagenesis of DNA. A class of mutations called recA(Prtc) confer protease activity constitutively and provide a tool for understanding how the RecA protein functions in both repair and mutagenesis. The recA(Prtc) mutants will be used to understand the nature of the mutagenic process that accompanies SOS repair as well as the spontaneous mutation process. The process of preferential mutation in physical proximity to the recA gene itself will be analyzed. These mutants also stimulate excision and transposition of the Tn5 transposon and potential mechanisms for this stimulation will be explored. The studies will be extended in an analogous way to a eukaryotic cell using a mutational approach. At least three mechanisms of repair are known that will be examined: photoreactivation, multiplicity reactivation (a recombinational mechanism), and an unknown mechanism defined by mutation that is possibly excision repair. Some of the tools developed in this work will be applied to a study of the photoinactivation of DNA in the presence of certain potential DNA binding compound such as urocanic acid present on human skin or potential photochemotherapeutic compounds like cis-bis(phenanthroline) rhodium chloride, an analog of "cis-platinum".